Method of manufacturing piston rings



Dec. 4, 1945. H. M. BRAMBERRY 2,390,417

METHOD OF MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheetl Dec. 4, 1945. H. M. BRAMBERRY 90,417

METHOD OF MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheet3 4, 1945. H. M. BRAMBERRY METHOD OF MANUFACTURING PISTON RINGS sSheeis-Sheet 6 Filed July 14, 1943 Dec. 4, 1945'.

H. M. BRAMB ERRY METHOD OF MANUFACTURING PISTON RINGS Filed July 14,1943 8 Shee'ts-Sheet 7 4, 1945. H. M. BRAMBERRY 2,390,417

METHOD MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheet 8Patented Dec. 4, 1945 OFFICE METHOD OF MANUFACTURING PISTON RINGS HarryM. Bramberry, Oak Park, Ill.

Application July 14, 1943, Serial No. 494,614

21 Claims.

This invention relates to the manufacture of steel articles of smallcross-section and particularly to the heat-treating and shaping of sucharticles composed of special steel.

It is the general object of this invention to provide a novel andimproved method of manufacturing steel articles of small section fromselected steel alloys possessing special properties and'taking advantageof these properties in such a way as to accomplishv either the shapingof the article; the imparting to the article of the required physicalcharacteristcs; the preserving of the contour of the article against theoccurrence of distortion; or selected combinations of these results.

Possibly the most important current application of my invention is inthe manufacture of steel piston rings, and, hence, I will give adetailed description of this method of piston ring manufacture as it isactually being successfully practiced in the fabrication of piston ringsfor military aircraft engines and engines for other equipment. It willbe understood, of course, that this is by way of preferred example, itbeing apparent that a wide variety of other articles may be fabricatedin a like or equivalent manner coming within the broad scope of myinvention.

I am aware that much previous work has been done in an effort toproperly treat, nitride, shape and otherwise process small section steelarticles. However, the particular technique or method herein presentedis believed to be new and novel over anything heretofore known.

Prior to the present invention, it is not believed that an article ofsmall sectionsuch, for example, as a piston ring--was eversatisfactorily made from steel by subjecting the same to a nitridingtreatment without the occurrence of such excessive distortions as torequire the removal of practically all of the effective nitrided case incertain areas in order to finish-machine the article to the requireddimensions. While I found that nitridng a steel article of small sectionprovided the same with the requisite case.

thickness and wear characteristics-as, for ex-' ample, in providing thewearing surfaces of a piston ring-I also found that this same nitridingprocess caused so much distortion in the contour of the ring as tonecessitate the removal of such a critical proportion of the casethickness as to approach dangerously close to the underlying corestructure in certain areas. Prior to the present invention, it has beenfound that in a high percentage of the resulting rings or other nitridedsteel articles of small section, the necessary removal of stock in orderto produce uniform dimensions has resulted in actually cuttingcrltically into the case, which, of course, renders that article or ringcompletely unsatisfactory.

It is a well-known fact that the aircraft engine builders have beenconfronted with increased power requirements year after year. Suchincreased power output has resulted in tremendous piston ring loads, somuch so, that the piston ring constitutes the limiting factor withrespect to further power increase if the same is to be realized withoutsuffering an appreciable reduction in life and efficiency. To meet theincreased power requirement and eliminate the piston ring as being thebottle-neck to further development, it was necessary to adopt new andreduced piston ring proportions, especially a much reducedcrosssectional area. Steel, having such high physical characteristics,has been found to meet the requirements, provided the load-carryingsurfaces can be so treated as to give a surface, as well as case andcore structure that can be loaded to the maximum without the attendantscuffing, scoring and non-compatible condition experienced with thegeneral run of quenched and drawn steels.

It is, of course, known that steel rings as well as cast iron rings maybe plated by the chrome process to provide a wear surface with a degreeof success. There are, however, a number of disadvantages which renderthis process incapable of producing rings which will operate under thesevere conditions which the rings of the present process are called uponto operate. In the first place, one of the most serious weaknesses inthe small section article such as a piston ring treated in accordancewith this known chrome process is the resulting hydrogen embrittlement.Also, extreme dimculty is encountered in effecting a uniformlydistributed thickness of chrome plate. In addition, when operated underdusty conditions, the plating soon wears away, which immediately resultsin non-compatible load-carrying surfaces. Therefore, chrome plated ringsfind limited usefulness where conditions are not as severe as thoseconditions under which it is contemplated that the present rings mustoperate.

It is an object of this invention to provide a method of fabricatingsteel articles of small section-particularly steel piston rings-of aselected steel having the property of growing when heat treated andtaking advantage of this growth to produce a more satisfactorystructure.

It is another object of this invention to provide an improved method andtechnique of nitriding steel art cles of small section-particularlysteel piston rings of small section-wherein the contour of the structureis preserved or held in such a manner as to prevent the occurrence ofdistortions normally resulting from such nitriding process.

generally the case and core relationship in each ring resulting fromsuch treatment;

It is a more detailed object of the present inwhich is to be heattreated in accordance with.

the present invention and from which a split piston ring is'subsequentlyto be made by cutting out a section therefrom in the so-called joint Iarea:

Fig. 2 is an enlarged broken away section view showing the joint areaand th manner of recessing the same prior to heat treatment;

Fig. 3 is an enlarged radial cross-section of a compression ring blankwith part of the ring broken away and indicating the uniform compositionof the metal and relative size of the section prior to ,being treated inaccordance with the present invention;

Fig. 4 is an enlarged radial cross-section view corresponding to Fig. 3but showing an oil control ring blank and indicating the substantiallyuniform composition of the metal prior to the ring being treated inaccordance with the present invention;

Fig. 5 is an axial section view of my improved male preshaping furnacefixture form, associatedloading ring member, and ring bending tool andguide;

Fig. 6 is a plan view taken substantially on the line 6-6 of Fig. 5 withcertain parts removed and others broken away;

Fig. '7 is a plan view taken substantially on the line of Fig. 5 showingan endless ring blank in firmly engaged position about the malepreshaping form; I

Fig. 8Vis a view similar to Fig. 7 but showing an endless ring blankbent inwardly in the joint area by means of the bending tool and guide;

Fig. 9 is a broken-away plan view partially in section takensubstantially on line 9--9 of Fig. 10 showing the complete furnacefixture with the rings in position firmly engaging the inner malepreshaping form and with the outer female nitriding form positioned inembraced relation thereabout and spaced slightly therefrom;

Fig. 10 1s an axial sect on view taken substantially on thelirie I 0-H]of Fig. 9;

Fig. 11 is a view indicating schematically the nitriding furnace withthe furnace fixture positioned within the furnace;

Fig. 12 is an enlarged broken-away section view corresponding to aportion of Fig. 10 and showing a group of endless compression ringblanks in close engagement with the wall of the inner male preshapingform prior to the heat treatment;

F g. 13 is a broken-away cross-section view corresponding to Fig. 12 butshowing the position of the endless ring blanks in .pressure' engagementwith the inner peripheral wall of the female nitriding form at the endof the nitriding heat treatment and growth process and indicating Fig.14 is a perspective-like view of the female furnace fixture nitridingform as the same appears after being removed from the furnace followinga heat treatment with the ring blanks in position exerting radialpressure against the peripheral wall of the female form requiring theopening of the fastening latch in order to eflect the release. of therings;

Fig. 15 is a schematic view of the endless, ring blank holding jig andthe grinding wheels for removing the stock at the Joint toprovide thefinished split piston rin Fig. 16 is a plan view 01a finished piston rinindicated schematically in closed position in the cylinder of aninternal combustion or other compression engine;

Fig. 1'1 is an enlarged cross-section view of a compression piston ringblank, such as shown on a smaller scale in Fig. 13 resulting fromtreating the blank of Fig. 3, and bringing out to advantage the detailsof the case and core relationship resulting from the present method ofheat treatment;

7 Fig. 17a is a view corresponding to Fig. 17 but showing the completedring section after the finishing operations have been performed;

Fig. 18 is an enlarged cross-section view of an oil control ring blankbringing out to advantage the case and core relationship resulting fromheat treating the ring of Fig. 4 in accordance with the presentnitriding method;

Fig. 18a is a view corresponding to Fig. 18 but showing the completedoil ring section after the finishing operations have been performed;

Fig. 19 is a plan view corresponding to Fig. 9 but showing analternative arrangement for practicing the present invention wherein thering blank is heat treated in the split condition thereof instead of theendless condition of Fig. 9;

Fig. 20 is an axial cross-section view of an internal combustion enginecylinder barrel or liner blank in position in a nitriding furnace formto be heat treated in accordance with the present invention; and

Fig. 21 is a broken-away section of a gear and nitrlding furnace formarranged for heat treating the gear in accordance with the presentinvention.

The present method involves essentially taking.

lowing the article to grow and exert pressure engagement against'theconfining form, I accomplish the very important result of preserving andsetting the contour of the article by preventing the occurrence of thenormally present distortions caused by the heat treatment.

My invention will first be described by way of preferred example asapplied to the fabrication and heat treatment of a steel piston ring.Referring to Fig. 1, a piston ring blank PRB is shown as comprising anendless circular ring, larger in diameter than the finished ring, to bemanufactured, by the amount of the selected length of stock to besubsequently removed and indicated at JSR, (joint stock removal).

The rings. the manufacture of which is herein described, are those thatI have built for the Wrl'ght-Aero-R-ISZO cyclone engine. The compressionring blank, cross-section of which is shown in- Fig. 3, has an initialoutside diameter IOD in the rough form of 6.494"i.001", requiring asubsequent joint stock removal JSR of 1.242" measured on the chord. The011 com trol ring blank, cross-section of which is shown in Fig. 4, hasan initial outside diameter of 6.589":.001", requiring a subsequentJoint stock removal of 1.565". Referring to Fig. 2,'it will be notedthat an arcuate concave section CS is removed from each side of the ringblank within the area. JSR and a tool and guide locating notch N isformed in the face of the ring centrally of the area JSR. The functionof the concave section is to provide a space for the formation of up-setprotuberances when the ring is subsequently pushed or bent radiallyinwardly as will 7 appear. Otherwise, the occurrence of suchprotuberances would prevent orinterfere with the fiat contact ofadjacent rings when placed side by side in the nitriding fixture to bedescribed.

The initial ring stock or blank for making a compression ring before thesame has been treated to provide the important case, core and otherphysical characteristics is shown in enlarged cross-section in Fig. 3.The section is indicated as being substantially uniform in structuralcomposition throughout. The compression ring initial radial thicknessCIRT is 0.1525: .0005" when made of drawn wire. The compression ringinitial width CIW after roughing of the sides to provide for gascirculation is .0715":.0005". In other words, the desired surfaceroughness is effected by honing or grinding each side to provide for therequisite circulation of gas between adjacent rings while in the furnacefixture as will appear. The corners of the blank are rounded with aradius CR of 0.012" to 0.015".

In Fig. 4 there is shown an enlarged crosssection of an oil control ringblank before the same has been treated to provide the important case,core and other physical characteristics. This section is likewiseindicated as being substantially uniform in structural compositionthroughout. The oil ring initial radial thickness OIRT is 0172510005"while the oil ring initial width OIW is .0'7l5:.0005" after roughing ofthe sides to provide for gas circulation as above described. The insidecorners of the blank are likewise given a radius ORI of .012" to 0.015"while the face portion is initially defined by a pair of convergingsides OCS terminating in a a known for arriving at and pr parin thisshape, any one of which may be employed.

I have devised a novel method or technique for readily providing theoptimum-shape of male preshaping and female forms, this subject matterbeing incorporated in a separate case.

Next, the ring blank PRB is pressed or bent inwardly in the areaopposite the Joint stock removal portion JSR by means of a uitable toolll. The male preshaping form I2 is provided with a fiat portion I5 andthe ring blank; is bent inwardly with reference thereto (Figs. 6 to 8)to provide for resilient action of the ring blank and to eflectconformity of the inner periphery of the ring blank against thepreshaping male form. Also of importance is the fact that this preventsfatal distortion thereof which would otherwise result from forcing thesame about a male preshaping form not provided with such flat portion.This preshaping male form I2 is given a concentric shape to that of theinner periphery of the female form I! (Fig. 9) but, of course, smallerin order to permit of the introduction of the ring blanks PRBtherebetween. With the ring thus in place about the male preshapingform. l2 and the female nitriding form I! placed in embracing relationthereabout (Figs' 9 and 10), there exists a slight clearance between theouter periphery of the ring blank PRB and the inner periphery of thefemale form l1 around the entire circumference; except in the area JSRopposite the fiat portion ii of the male preshaping form, which is notimportant since this section of the ring will later be removed. Forexample, in the present case involving the manufacture of a compressionring having a finished closed ID of 6.125", a final radial thickness of0.150" and final.

as Nitralloy N because of the unusual properties of this metal and thenovel and unexpected manner in which the same responds to the methodsteps herein disclosed.

Nitralloy N is a commercially available alloy steel. The composition ofNitralloy N comprises- C (Carbon) ..1 per cent" Mn (Manganese) do040-.70 Si (Silicon) maximum 0.30 A1 (Aluminum) per cent 1.10-1.40 Cr(Chromium) do 1.00-1.30 Mo (Molybdenum) e do 0.20-0.30

. Ni (Nickel) do 3.25-3.75

a ring guide ID by a reciprocable loading tool I and the balance iron,except, of course, :for impurities. The exact extent to which variationsin this composition of Nitralloy may be permissible has not been exactlydetermined. However, the product supplied, for example, by the AlleghenyLudlum Steel Company, or any of the other sources and specified asconforming with the above composition has, in my experience, been foundto produce rings of the desired characteristics. I do not claim anyinvention in this particular metal alloy per'se but do claim theherein-disclosed method of manufacturing articles' from this alloy, aswell as from others having the essential common characteristics.

with the furnace fixture containing the rings in position within thefurnace 19 .as indicated in- Fig. 11, the same is first subjected to apurging Y which comprises blowing gaseous ammonia therethrough at arelatively high rate and at a soaking temperature of the order of 925:5"F. for a period of about two hours. During this time a heavy ammoniaflow is maintained, with the result that ammonia disassociation isprevented as well as preventing the beginning of nitriding action on thering blank. Of particular importance is the fact that during thissoaking period the rings are sufficiently relieved to effect a set inthe shape thereof to the shape of the male preshaping form l2 aboutwhich the same have been previously introduced. The accomplishment ofthis shaping and setting is believed to be the result of an increase inthe physicals of the blank as evidenced by an appreciable increase inRockwell hardness, which occurs in the Nitralloy blank as a directresult of heating in the temperature range of from 900 to 1000 F. Thisincrease of the physicals of the Nitralloy N blank under theseconditions appears to be a unique characteristic of this Nitralloy "Nand is not present in any other known alloy, at least to this markeddegree. The heavy ammonia flow serves the purpose of preventingnitriding and its accompanying growth until the Dreshaping or setting ofthe ring blank to the shape of the male form has been accomplished. Thispreshaping and setting is'very important because the male furnace formhas a concentric shape to that of the female furnace form. The ringblank having thus been first shaped and set about the male form willsubsequently grow away from I the male form and into uniform pressurecontact with the inner peripheral wall-of the female form.

The actual preshaping temperature as well as soaking time employed is aresult of experiment in the manufacture of rings and may be variedsomewhat depending upon circumstances. It is to be noted, for example,that a somewhat higher temperature such as a temperature of the order of975 F. approaching more closely th critical or normalizing temperatureof 1300 F., would effect a more complete normalizing and setting toshape. Such higher temperature would be accompanied by the growthphenomenon where subjected to ammonia gas and hence would defeat thepreshaping and setting to the shape of the male form. Where wesubsequently nitride and grow a ring blank away from the male form andinto contact with the female form as herein contemplated, it has beenfound that the degree of normalizing and setting to shape accomplishedat a relatively low temperatur of the order of 925 F. is suflicient,

As will be pointed out in another case directed to the broad aspects ofshaping by heating and normalizing, I have found that by carrying thistemperature higher in a non-oxidizing atmosphere a somewhat lessexpensive but less eflicient ring may be manufactured suitable forsubsequent use in a plating process such as that commonlyknown as chromeplating. No complete detailed description of the several alternativetechniques of preshaping in a non-oxidizing atmosphere without actualgrowth will be described herein as the present case is primarilydirected to the unusual phenomenon of growth in confinement.

Following the above preshaping heat-when the more important growthln'conflnement portion of my process'is to be practiced as hereincontemplated-the rings are'next subjected to a normal nitridingatmosphere for an additional time period of about thirty-five hours at995:t5' F., within the furnace outside the retort (see Fig. 11) Thisresults in the ring blanks PRB growin away from the male form l2 andinto contact with the embracing inner peripheral wall of the concentricfemale form II, which is a very important aspect of my invention.

While the above temperature ranges and nitriding time are employedbecause of the particularly satisfactory results obtained therewith. itwill be understood that in the broader aspects of my invention Icontemplate other temperatures coming within the nitriding range as wellas other nitriding time periods so long 'as the same are effective toproduce the herein disclosed growth in confinement.

By taking advantage of this growth characteristic of the metal under thenitriding conditions and allowing the ring blanks to grow into contactwith the embracing female form, a most important result is obtained. Thefemale form I'l, being made of substantial cross-section, retains itsidentical shape, and when the relatively small section ring blanks PRBare made to grow into pressure contact therewith, the surface contour ofthe ring is thoroughly preserved and the overall shape of the femalenitriding form attained by the ring blank resulting in a highlysatisfactory ultimate ring, notwithstanding the relatively small sectionof the blank involved. The furnace fixtures are made of 2330 S. A. E.(Society of Automotive Engineers) steel and are so completelystabilized, as well as protected by electroplated surfaces, that theyretain their shape over along period of operation and many heat cycles.

Particular attention is drawn to an unusual and phenomenalcharacteristic of Nitralloy N when treated as described above. From thevery beginning of the heat treatment there occurs an appreciableincrease in the physicals of the core. This is not true of any otherknown alloy. In the case of the present small section ring, thisincrease in physicals of the core is particularly significant in that itmakes the core a suitable carrier for the outer hard case.

Referring further to the furnace fixture (Figs. 5 to 14) the ring blanksPRB are held edgewise in a. flat parallel condition by a flat andparallel shoulder 22 provided therefor on the male form i2 at oneextremity and by a cover 23 at the opposite extremity thereof havingthereon a parallel surface 24 in contact with the topmost ring blank.This cover is held or clamped in position by the use of Bellevillespring washer l8 having a constant spring rate and being adjusted to thedesired pressure by a through bolt 25 and nut 26.

The surrounding female form I1 restson and floats upon the base 21 ofthe fixture being provided with positioning means which may be in theform of a pin 28 carried by female form I! and riding in a radiallyextending keyway 29. This arrangement allows for the free movement ofthe female form H to adjust itself with reference to the growth of thering blanks, thus assisting in providing uniform pressure engagement ofthe ring blanks with the female form throughout the inner circumferencethereof. A locking pin 3| passes through the head of bolt 25 and intobase 21 preventing relative rotation upon the turning of nut 26.

The female form I! is preferably composed of a split ring of many timesthe cross-section of the ring blank or blanks to be grown into radialpressure engagement therewith. I have found that a female form I!composed of 2330 SAE steel, and nickel plated, is satisfactory formanufacturing the present rings when given a radial thickness of theorder of from five to ten times that of the rings to be nitridedtherein. The

axial width will depend upon the number of rings to be treated in asingle heat, the female form 11 shown in Fig. 14 having a width of theorder of 2" and having a capacity of twelve rings. 1

For accurately maintaining the inner peripheral circumference, the endsof the female ring form ll are provided with a drilled and reamed holeextending in the axial direction, through which the ring is sut at 32and into which drilled hole is placed a dowel 33. The female ring endsare arranged to be fastened against this dowel 33 by means of a pivotallatch 34.

When the nitriding treatment has been completed, it will appear that therings may be readily removed from confinement within the female form 11by first releasing latch or fastening means 34. This is followed byemploying a wrench or wedge inserted in the radially extending slot ofthe female form i! for slightly opening the same.

It is particularly imp rtant that the nitriding atmosphere be circulatedto all of the surfaces of all of the ring blanks substantially uniformlyin order that the depth of the resulting nitrided case below the severalsurfaces may be made substantially uniform. In a piston ring this isimportant in order to provide the proper physical male form, horizontalcirculating channels 33 (see Fig. 12). Male form I2 is likewise providedwith axially extending recessed gas channels 42. Base 21 is providedwith gas passages 43 communicating with the lower extremities of chan'-nels 31 and 42 as well as with a gas passage 44 communicating with thetop extremity of passages 31 and 42 through the interiorof male form l2by way of opening 46 and space 41 between male member I2 and cover 23.The spacing 41 provides for firm pressure contact between the top ringand the cover member 23. This is very important in accomplishing therequ site substantially perfect flatness of the resulting rings. Thiscover is also provided with a gas opening 48. A pin 43 passes throughcover 23, male member l2 and base 21 functioning to maintain the properrelation therebetween.

When the endless ring blanks are removed from the female form I!following the above nitricling growth treatment it is found that across-section therethrough presents a very definite structural picture.Referring to Fig. 17, a cross-section of a compression ring is shown ascomposed of the inner core, the adjacent transition zone CHTZ, and theprime or hard case CHPC and an outer matted layer CHM. The transitionzone CHTZ has a general depth of the order of .0035" to .0065". Theprime hard case CHPC thickness is of the order of .008" to .011".Following the removal of the endless rings from the female form I! asnoted above, the rings are lapped or finished-honed on each side toremove characteristics including: the proper free joint opening when thesection of stock JSR has been removed; the proper radial pressurecharacteristics including the radial point pressures, that is, radialpressures at the ends of the ring, which pressures must be held withinclose limits; the

proper depth of case below the cylinder wall engaging face; the propertensile strength, and hence, the proper fatigue characteristics; theproper conformity with the contour irregularities of the cylinder wallsat high temperatures and pressures; and full control of flatness. Toprovide this uniform exposure to the nitriding gases, the innerperipheral wall of female form I! is fabricated with a plurality ofaxially extending recessed gas channels 31.. These gas channels are ofsuch small circumferential extent as not to make any impression upon theouter periphery or face of the rings when the same exert radial pressurethereagainst. I have found that about 17 of these gas channels 3'!having a circumferential length each of about 0.062" and a depth ofabout 0.031" are suflicient to provide ample circulation to the ringblank faces. Particular attention is directed to the fact that thecompression ring blanks are formed with a small radius CR of .012" to.015" at each extremity of the ring face with the result that adjacentrings define with the inner peripheral wall of the female form-duringthe final portion of the heat-forward gas circulating channels 38 (seeFig. 13).

As pointed out above, the side of the ring pin 3| passes through thehead of bolt 25 and blanks has been given thedesired surface roughnessproviding for gas circulation therebetween.

When the heat is firstbegun andthe rings firmly engage the malepreshaping form I 2 of the inner radiused corners CR define with the thematted surface CHM and to reach the underlying structure of the primehard case CHPC removing as little of the latter as is necessary in orderto reach a uniform surface. It is found necessary to remove only about.0005" to .001" from each side of the ring blank in order to obtain auniform hard case CPCS having a depth of the order of .007" to .010"below the surface on each side.

Next the rings are "set up to remove approxiured along the outer chordas indicated on Fig. 15. In accomplishing this the rings are held in afixture 54 while the grinding wheels 55 cut out the requisite sections.Finally the rings are codirectionally finish-honed on the cylinder wallengaging surface to remove the matted layer and to leave as much aspossible of the underlying prime hard case. As a result of my novelprocess, I am able to leave a prim hard case depth CPCF on the cylinderwall engaging face of the present rings of the order of .0055" to.0085", by honing away CY to a depth of only about 00025.. A case depthof .0040" to .0090" is within acceptable limits to produce asatisfactory ring, when the inaccuracy of potting up and honing isconsidered.

Referring now to Figs. 18 and 18a, showing cross-sections of an oilcontrol ring, a similar structural arrangement is presented, the prefixletter 0 being used to differentiate the oil control ring from thecompression ring C. The principal difference is in the cylinder wallengaging surface.

As will be noted from the following comparative analysis of theresulting compression ring and oil ring, the prime hard case of the oilring in the nose area is somewhat deeper as a result of the particularconverging construction of this nose. This is a distinct advantage sincethe unit pressure of this reduced surface in engagement with theembracing cylinder wall is greater with a consequent slightly increasedrate of wear over that which is present in the wider face of thecompression ring- The following gives a comparison of the compressionring blank before heat treatment (Fig. 3), after heat treatment (Fig.1'7) and in the finished form (Fig. 17a) (1) Compression ring blankinitial outside diameter,,CIOD 6.494 .001 Compression ring blank initialoutside circumference mounted on male form, CIOC 20.403" Compressionring blank initial radial thickness, CIR.T 0.1525":.0005 Compressionring blank initial width when rough finished on sides to provide for gascirculation b e t w e e n sides, vCIW .0715":.0005 Compression ringblank heat treated outside circumference, CHOC 20.465"

Inner circumference of female form 20.453" CHOC (20.465") minus CIOC(20.403")=0.062" restrained or confined in fixture average growth.Compression ring blank heat t r e a t e d radial thickness, CERT.1530":.0005 CHRT (0.1530") minus CIR'I' (0.1525") =0.0005" averagegrowth. Compression ring blank heat treated width, CHW .072:!:.0005" CHW(0.07175" mean) minus CIW (0.07125" mean) =.0005" a v e r a g e growth.Compression ring blank heat treated prime hard nitrided case, CHPC.008"to .011" Compression r i n g b1 an k heat treated depth oftransition zone, CHTZ--- .0035" to .0065" Compression ring finished,cut, closed, outside (1 i a m e t e r, CFOD 6.125":.001 Compression ringfinished, cut closed, radial t h i c k n e s s, CFRT .150"- -.0015 CHRT(0.153" mean) minus CFRT (0.150" mean)=.0030" maximum removed infinishing.

Although this difference is .0030", we hone a maximum of only .0025"which still keeps within a finished dimension of .150i.0015".

Thus, in finishing the cylinder wall contacting face of the rings, Iremove .002" to .003". This portion that is removed is composed of about0.0005" of matted surface and a thin portion of the underlying hardnitrided case.

Compression ring finished, cut,

closed, width, CFW .070"- -.0005" CHW; (0.07175" mean) minus CFW(.070")=.00175" mean total removed.

Thus, in finishing the sides of the ring, I remove about .0009" fromeach side. This is composed of about .0005" of matted surface andunderlying hard, nitrided case.

Compression ring finished, prime hard nitrided case depth remainingbelow the face, CPCF .0055" to .0085" Compression ring finished, primehard nitrided case depth remaining below each side, CPCS .007"-.010

The difference in the finisheddepth of case between the cylinder wallengaging face and the sides is explained by the fact that it is notpracticable to effect the same degree of perfection in finishing thecylinder wall engaging face as in finishing the sides. This is accountedfor in part by the fact that the rings after being cut must be assembledin a special fixture and then transferred to an arbor on which they areheld while being honed. This cannot be accomplished more perfectly thanto within .001" to .002".

The following gives a comparison of the oil ring blank before heattreatment (Fig. 4), after heat treatment (Fig. 18) and in the finishedform (Fig. 18a) radial thickness, OHRT .173010005" OHRT (.1730" mean)minus OIRT (.1725 mean) ='.0005" average growth. Oil ring heat treatedwidth,

OHW .0720"- -.0005" OHW (.0720" mean) minus OIW (.0715" m e a n)=.0005"a v e r a g e growth.

responding to the depth of casein the compression ring.

The significance of the above outlined small dimensions will now beapparent. By allowing the ring blanks to grow into pressure engagementwith the surrounding nitriding female form, I thus prevent any irregulardistortions in the contour of the rings. Since there are no undulationsor irregularities in the contour, it thus becomes necessary to removeonly the very minimum of the exterior surface of the heat treated ringsin order to reach the underlying prime hard case structure required forcontact with the piston groove and with the cylinder wall. For the samereason the underlying prime case is left with the maximum depth, andthis depth is uniform throughout any particular surface.

A starting blank in th form of an endless ring has been found to havedefinite advantages, including: (1) it can be semi-finished by passingthrough double-disc grinders with no free ends requiring control; (2)full control of the radial pressure at the points, which is highlydesirable,--

and which cannot be accomplished within the required close limits byany'manufacturing technique heretofore known to the art with a splitring; (3) the Sides of a piston ring with surfaces as hard as that afterbeing subjected to the nitriding process can only be reduced in width bysome sort of abrasive operation and during this operation, a solidendless piece can be more accurately finished to dimension than can asplit piece; and (4) the last and final operation of removing thesection JSR at the joint area can be done in group order which isadvantageous to production and the maintenance of close tolerance in therelation of the ring ends.

While, as noted above, an endless ring has been disclosed as the blankto be subjected to the nitriding process, and this is the preferredprocedure, it is also contemplated that this invention covers the use ofa split ring blank (see Fig. 19). In practicing the invention with asplit ring blank, the same may be assembled with the ends engaging theproper dimensioned feather 56 within the fixture so as to provide theproper percentage of growth to take place before the ring completelyfills the female form and exerts the required pressure thereagainst.

The same fundamental reaction can be accomplished without the use of afeather by using a split ring of the correct circumference leaving aclearance in the joint area to provide for the proper percentage ofgrowth before the ends come into contact to create the desired'pressureagainst the restraining female form. In addition, the

ends of such a ring may be so angled as to cause the pressure at theperiphery of the ring directly at the joint to become either negative orpositive.

I In manufacturing a ring in accordance with the latter practice, itwould be necessary to remove the ends in a similar manner to the removalof the joint areajrom the first and preferred endless blank. a

Different percentages of the total growth for pressure engagement withthe female member may be used following actual contact of the outerperiphery of the ring with the female member, ranging anywhere from 20%to 40% of the total growth. It has been found, for example, that aparticularly satisfactory Nitrallo N steel piston ring of 6.125" O. D.,0.150" final radial thickness and 0.070" final width is obtained whereseventy per cent (70%) of the possible growth of the ring is allowed totake place before the outer periphery or face of the ring exerts anappreciable pressure against the surrounding wall of the female member.Thirt per cent of the total growth is thus employed after actualpressure engagement with the female member for preserving the contourand flatness of the final ring and imparting to the ring other improvedproperties flowing from this process. a

By following the above step of first forming the ring over theconcentric male preshaping member [2, there results the very noveladvantage of having the ring grow substantially uniformly and identicallinto engagement with the female member I! throughout the circumference,with the exception, of course, of the joint area JSR. This effects asubstantially uniform pressure between the ring and the inner peripheralwall of the female member and substantially complete contact therewith.This is clearly distinguishable from taking a circular ring, compressingthe same along one diameter to form aparabolic-like shape and insertingthe same into the female member and following this with the ispractically that of the engine cylinder diameter for which size the ringis intended. A further and ver marked defect in this irregular growthinto contact with the female-form would be the fact that non-uniformradial pressures would be produced between the ring face and the femalemember, some of these pressures being so high that the nitridingatmosphere would be unable to gain access to the ring while in otherplaces the atmosphere would gain access, thus resulting in a non-uniformnitrided case. It is essential that the small section steel ringinvolved in the nitriding operation absorb the nitriding atmosphereuniformly over all surfaces so as to prevent bi-metal effect which wouldresult in not only a distorted contour but an out of flat conditionwhich cannot be corrected.

If the case is appreciably thicker on the inner periphery of the ringthan on the outer, when the joint pieceJSR is cut out, the unstablecondition will immediately evidence itself by the fact that the ringopens because of the high stress on the inner periphery; conversely, theopposite result takes place if the case on the outer periphery is ofgreater thickness than on the inner.

The same result as that obtained by the use of the male preshapingmember can be obtained to lesser degree by pre-bending the ring blank inthe area. of from three to nine o'clock to a small radius in the areasof two and ten oclock before placing the same in the female member.

The relation between the total nitrided case cross-sectional area andthe total core cross-sectional area is critical in articles of smallsections and has a definite relation to the total length of the piece,such, for example, as the herein-disclosed piston ring. This relationdetermines the amount of growth possible per unit of circumferenceduring the nitriding furnace time cycle selected thus to obtain therequisite pressure against the female form and effect the desiredphysical characteristics. A satisfactory prime hard case thickness, forexample, in an aircraft nitrided steel piston ring of an 0. D. 6.125"has been found to be .0055 to .0085", this being arrived atbymanufacturing and engine operation requirements and conditions madenecessary in order to obtain the desired high fatigue value andresiliency for high output, high-speed internal combustion engines.

It is believed that no one has heretofore appreciated the significanceof the optimum relationship between the case and core area of a nitridedsteel piston ring and how to secure the same. From said relationship anaccurate value can be established from which the percentage factor fordetermining the percentage of the total growth can be selected forpredetermining 'ing furnace fixture growth of a steel article or steelring blank may be determined, the necessary specimens are subjected to anitriding heat, being measured accurately before and after the heat andfrom their individual growth an average value of growth is establishedper unit of length. With the.,length of the female interiorcircumference known, the required length or diameter of a ring blank ofthe same steel as the specimens can be easily determined from the valuepreviously established by specimen, in order to employ the properpercentage of the growth of the ring to exert the required pressure onthe interior of the female form necessary to preserve the shape of thefinished ring without adversely affecting the penetration of the gasesto the face thereof.

My invention has been described above in considerable detailinconnection with the specific manufacture of piston rings in order toteach,

with the requisite completeness, how to practice this important process.From this teaching it is believed that the application of my inventionto the manufacture of other articles involving like problems will beapparent.

Referring to Fig. 20, one example of an alternative application of myinvention is indicated schematically. A Nitralloy N steel cylinder linerblank 51 is shown in position in spaced relation to the embracingfurnace fixture 58 prior to heat treatment. As in the case of theabovedescribed piston ring manufacture, the assembly will be placed in anitriding furnace, such as l9 shown in Fig. 11, subjected to a nitridingatmosphere and selected temperature cycle for a selected time. Thecylinder blank 51 will .be caused to grow into radial pressureengagement with the surrounding wall of fixture 58 and this pressureengagement will be utilized to preserve the contour of the relativelythin sectioned cylinder blank during the formation thereon of therequisite nitrided case. Subsequently, the cylinder 51 is removed fromthe fixture, honed and otherwise finished for installation in an engine.

As above emphasized, the fact that the cylinder blank is preserved orheld against distortion during nitriding it is only necessary to machinedeep enough'into the working surface of the cylinder to remove thematted surface therefrom, thus leaving a uniform depth of prime hardnitrided case of the order of .006" to .009". As in the case of a pistonring, the prevention of the formation of distortions in the contourmakes this result possible.

While furnace fixture 58 is not disclosed in detail, it will be apparentthat the same can be built in an equivalent manner to that of femalemember I! of furnace fixture l3. This may include splitting the form asindicated at 58a, as

well as providing a cover member 58b, Belleville spring washers 58c andfastening cap screws 58d. Referring to Fig. 21, there is indicatedschematically the manner of practicing my invention in nitriding a steelNitralloy N gear 6|. Gear BI is indicated in spaced relation to theembrac- 62 prior to the growth of the gear into pressure engagement withthe" form in a nitriding atmosphere. While the furnace fixture forreceivin ear 6| has not been shown in detail, it will be apparent thatthe same may be constructed in a similar manner to the above furnacefixture. This may include a plurality of separable segments H and an.embracing split ring Ila. This ring may be provided with a fastoningmeans, such as that shown in Fig. 14.

Although I have described my invention in usually novel properties inconnection with Nitralloym because of its unpracticing this method, itis to be understood that in the broader aspects of my invention, 1 alsocontemplate the use of any metal or alloy having the requisiteproperties. I

While my invention has otherwise been described in connection withcertain specific embodiments thereof, it is to be understood that in itsbroader aspects I consider this invention to be basic and not limited tothe specific examples given, the same being defined by the appendedclaims.

I claim:

1. The method of manufacturing articles having at least one relativelysmall cross-sectional dimension and composed of a steel alloy having theproperty of growing when subjected to a nitriding heat treating process.which method comprises confining a blank from which said article is tobemade in a furnace fixture having an internal contour corresponding tothe external contour of the final article and being slightly larger thanthe blank, subjecting said blank to a nitriding atmosphere fora selectedtemperature and time cycle, employing the growth of said blank to exertpressure against said fixture, said pressure being effective to hold thecontour of said blank and prevent the occurrence therein of distortion,and removing said blank from said fixture.

2. The method of manufacturing an article of small section of a steelhaving the property of growing or elongating when nitrided, whichcomprises subjecting a substantially circular metal blank, from whichsaid article is to be made, to a nitriding atmosphere within a confiningform or fixture which is larger than the blank and using the growthproperty of the-metal, due to the nitriding thereof, to effect radialpressure engagement with the form for preventing the inherent distortiontendency of said article and subsequently removing the article from theform and employing the same for some function outside of said form.

3. The method of manufacturing a steel article having at least onerelatively small cross-sectional dimension of the order of 0.030" to0.l30", said steel being an alloy having the property of growing orbecoming elongated when subjected to a nitriding treatment, said methodcomprising confining a blank from which said article is to be made in afurnace fixture having an internal contour corresponding to the externalcontour of the final article and being slightly larger than the blank,subjecting said blank to a nitriding atmosphere for a selectedtemperature and time cycle, employing the growth or enlargement of saidblank to contact and exert pressure against said fixture, thus toprevent the inherent distortion tendency of the contourof said blankduring the nitriding operation and subsequently removing the articlefrom the form and employing the same for some function outside the form.

4. The method of heat treating a substantially circular Nitralloy Nsteel article having such a small section as to normally result in thecontour thereof becoming distorted when subjected to a nitridingprocess, which method comprises confining said article in a furnacefixture having a formed interior corresponding to that of said articlebut being slightly larger and permitting of the ready insertion of saidarticle therein, said fixture being of a sufllciently rigid constructionas notto be distorted by a ni-tridins process or the pressure of thegrowth of said article, sub-- jecting said assembly to a nitridingatmosphere for a predetermined period of time and at a predeterminedtemperature and employing the radial growth characteristic of said metalarticle during a predetermined percentage of the total growth thereofwhen subjected to said nitriding atmosphere to effect radial contact ofsaid article with said formed interior and the exertion of sufiicientpressure thereagainst to cause said article to maintain conformity withsaid formed interior andprevent the occurrence of irregularities in thecontour of" said article, and subsequently removing said article fromsaid formed fixture and adapting the same for performing some functionexterior to said fixture.

5. The method of heat treating'a nitridable steel piston ring blankwhich includes: placing said steel blank in a confining fixture oflarger internal dimension than the external dimension of the blank,subjecting the same to a nitriding atmosphere for a selected temperatureand time cycle, employing the growth characteristic of the nitridablesteel while being nitrided to effect contact with and to exert pressureagainst said fixture for preventing the inherent distortion tendency ofthe contour of said ring and subsequently removing the ring from thefixture and adapting the same for use in a cylinder.

6. A method of heat treating a Nitralloy N steel piston ring blank whichincludes placing a ring blank of relatively small section in a furnacefixture arranged to confine the external radial periphery of the ringagainst growth beyond a predetermined amount thus to employ the growthcharacteristics of the nitridable ring blank while being nitrided forexerting pressure against said fixture for holding the same against theinherent distortion tendency of the contour thereof, subjecting the ringto a nitriding atmosphere for a predetermined temperature and time cycleand subsequently removing the ring blank from the fixture and performingthe necessary finishing operations thereon to produce a completed pistonring.

.7. The method of manufacturing a split piston ring which includesplacing an endless Nitralloy "N, steel ring blank of relatively smallcrosssection in a confining furnace fixture, subjecting the same to anitriding atmosphere for a selected temperature and time cycle,employing the radial growth characteristic of the Nitralloy while beingnitrided to effect pressure against said fixture for preventing theinherent distortion tendency of the contour of said ring, subsequentlyremoving the ring blank from the fixture, finishing the sides of saidring blank, then cutting out a predetermined section of stock to providea selected free joint opening, closing the ring on a holding mandrel andhoning the face of said ring to remove the matted surface therefrom andprovide a prime hard nitrided case having a uniform depth below saidworking surface of the order of .0040" to .0090".

circumference of saidring blank, subjecting the ring to a nitridingatmosphere for a selected temperature and time cycle, employingthegrowtn or elongation characteristic of the Nitralloy N when subjectedto the nitriding treatment. for exerting radial pressure against theparaboliclike inner peripheral wall of said furnace fixture for holdingsaid ring against the inherent contour distortion normally presenttherein when subjected to such nitriding process, subsequently removingthe ring from the fixture, finishing the ring blank on both sides toremove the matted surface and provide a prime hard nitridedcase ofuniform depth, cutting out a selected, length of stock from the jointarea, closing the ring on a holding arbor and honing the cylinder wallengaging surface to remove the matted surface and provide a hardnitrided working face with a uniform hard prime case depth of the orderof .0040" to .0090".

9. The method of manufacturing a finished steel piston ringhaving anoutside diameter when installed in a cylinder of the order of 6.125", aradial thickness of the order of 0.150", a width between sides of theorder of 0.070", which method comprises employing an endless Nitralloy"N steel ring blank of an outside circumference in rough of the order of20.403", a radial thickness of the order of 0.1525 plus or minus .0005",a width between sides of the order of 0.0715": .0005", placing saidendless blank in a nitriding female furnace fixture form having an innerperipheral contour parabolic-like in shape and having a circumferencegreater than the external circumference of said endless ring blank by apredetermined amount, subjecting the ring blank to a nitridingatmosphere for a predetermined temperature and time cycle, employing thegrowth of elongation characteristic of the Nitralloy N for exertingradial pressure against said parabolic-like .inner periphery of saidfemale niltriding form during a selected percentage of the growth ofsaid ring blank for the purpose of preventing the occurrence ofirregular distortions in the contour when subjected to such nitridingtreatment, subsequently removing the ring blank from the fixture,finishing the sides thereof to remove the matted surfaces and provide auniform predetermined depth of hard prime case below said surfaces,cutting out a selected length of stock from the area. at twelve oclockor the commonly referred to joint area, closing said ring about a honingmandrel and honing the cylinder engaging surface thereof to remove thematted surface and provide a uniform depth of prime hard nitrided caseof the order of .0055" to .0085".

I 10. In a furnace fixture for nitriding articles composed of anitridable steel alloy having the property of growing or becomingelongated when subjected to a nitriding heat treatment, a nitridingfemale furnace form having an inner peri- 8. The method of manufacturinga split piston phery corresponding in contour to the external peripheryof the article to be nitrided therein, but being of such size as topermit of the introduction of the article therein prior to nitridingtreatment, said fixture being eifective to resist the pressure exertedby growth of said article thereagainst thus to preserve the contour ofthe ar-' ticle against the formation of irregular distortions in thecontour thereof, said fixture having" means providing for the release ofsaid article fol-- lowing the nitriding treatment. I I 11. In a furnacefixture ar an me ts a posed of anitridable steel alloy having 'the ith tmale furnace form having a predetermined shaped inner peripherycorresponding in contour to the external periphery of the completedarticle to be nitrided, but beingof a larger size to permit the purposeof removing said ring following the heat treatment.

12. The structure defined in claim 11, wherein said fastening andreleasing means comprises a latch.

13. In a, furnace fixture assembly for nitriding nitridable steel pistonrings having the property of growing or enlarging when subjected to anitriding heat treatment, a nitriding female furbolic-like incross-section and having aninner circumference slightly larger than theexternal circumference of an endless piston ring blank of nitridablesteel to be treated therein, a male prev nace form having a formed innerperiphery parashaping furnace form having a'formed external peripheryconcentric in shape to the paraboliclike inner periphery of said femalemember but spaced therefrom by a predetermined amount.

means for introducing endless ring blanks over said male memberincluding a guide member leading thereto and effective to change thecircular ring blank to the parabolic-like shape of the male member, saidmale member being effective to pre-shape the ring blank during theheating operation following which said ring is subsequently grown awayfrom said male memher and into pressure contact with said female memberinner periphery for preserving the contour of said ring against theformationtherein of irregular distortions.

14. The furnace fixture assembly defined in claim 13 being furthercharacterized by the provision of axially extending gas communicatingchannels formed in the peripheral wall of said female form and said maleform.

15. The furnace fixture assembly defined in claim 13 being furthercharacterized by the provision of axially extending gas communicatingchannels formed in the peripheral wall of said female form and said maleform, means defining a base plate supporting said male and female forms,said base plate being provided with a ra dially extending guide channel,an axially extending pin protruding from said female form and arrangedto be guided in said radially extendin channel, a cover for said maleform having a peripheral shoulder arranged to engage one side extremityof an outside ring, a bolt received centrally of said assembly, aBelleville type spring washer and a fastening means therefor for holdingsaid assembly together and maintaining a predetermined end pressure onsaid rings.

16. In a furnace fixture assembly for nitriding split nitrldable steelpiston rings having the property of growing or becoming elongated whensubjected to a nitriding heat treatment, a nitriding female furnace formhaving a formed inner periphery corresponding to the external peripheryof a split piston ring when free, and having an inner circumferenceslightly larger than the external circumference of said split pistonring when free plus the length of the free Joint opening, means defininga feather or filler arranged to be interposed between the ends'of thesplit rings to be treated in said female form, whereby the introductionof nitriding atmosphere about rings so confined and at nitridingtemperature for a selected period of time is effective to cause saidrings to grow into radial pressure engagement with saidfemale form andthus hold the contour of said rings in such a manner as to prevent theoccurrence of distortions therein.

1'7. The method of manufacturing a cylinder which includes placing atubular cylinder blank of a nitridable steel having the property ofgrowing when nitrided in a nitriding female furnace form having a formedcircular inner periphery slightly larger than the external circumferenceof said cylinder blank, nitriding said blank in a nitriding atmospherefor a selected period of time and at a selected temperature to effectthe growth of said blank to contact with and exert radial pressureagainst said furnace form, said radial pressure being effective to holdthe contour of said blank and prevent the occurrence therein ofdistortions, and removing said blank from said form.

' 18. The method of manufacturing a Nitralloy N piston ring from asplit, normally circular piston ring blank, which comprises spreadingthe free ends of said blank 9. selecteddistance apart, confining theblank within a furnace fixture, unlformly subjecting all surfaces ofsaid blank to heat and a nitriding atmosphere to a'temperature below thecritical temperature of the Nitralloy N, for a selected period of time,and using the growth property of the metal due to nitriding thereof toeffect radial pressure engagement with the interior of the fixture.

19. The method of manufacturing a Nitralloy N piston ring from a split,normally circular piston ring blank, which comprises placing said blankin a confining fixture of larger internal dimension than the externaldimension of the.

blank, spreading the free ends of the blank a selected distance apart,subjecting all surfaces of said blank to a nitriding atmosphere for aselected temperature and time period, and employing the growth of theblank while being nitrided, to contact with and exert pressure againstsaid fixture to prevent distortion of the blank.

20. The method of heat treating a plurality of nitridable steel pistonring blanks, which comprises roughin the sides of said blanks to adesired degree of roughness, stacking said blanks with the roughenedsides of adjacent blanks in contact in a confining fixture of largerinternal dimension than the external dimensions of the blanks,subjecting the blanks to a nitriding atmosphere for a selectedtemperature and time cycle, the roughened sides of said blanks insuring,nitriding' of both sides of the blanks,and employ. ing the growth ofthe blanks while being nitrided, to contact with and exert pressureagainst said fixture to prevent distortion of the blanks.

21. The method of heat treating a. plurality of nitridable steel, splitpiston ring blanks, which desired degree of roughness, stacking saidblanks 2,sao,417

with the roughened sides or adjacent blanks in contact within aconfining fixture of larger internal dimension than the externaldimensions of the blanks, spreading the free ends or said blanks apredetermined distance apart, applying a selected yieldable load to thesides oi said blanks to maintain the same in normal flat condition,uniformlysubiecting all surfaces or said blanks to a nitridingatmospherefor a selected temperature and time cycle, the roughened sides of saidblanks insuring nitriding 01 both sida of all of the blanks. andemploying the growth of the blanks while being nitrided, to contact withand exert pressure against said fixture to prevent d18- tortion oi theblanks.

, HARRY M. Y.

